An electric power steering apparatus (EPS) which assists and control a steering system of a vehicle by means of a rotational torque of a motor, applies a driving force of the motor as a steering assist torque (an assist torque) to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus performs feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of a duty ratio of pulse width modulation (PWM) control.
A general configuration of the electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack-and-pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1 and a steering angle sensor 14 for detecting a steering angle θ, and a motor 20 for assisting a steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key (IG) signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist (steering assist) command on the basis of a steering torque Ts detected by the torque sensor 10 and a vehicle speed Vs detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 for the EPS by means of a voltage control command value Vref obtained by performing compensation or the like to the current command value.
Moreover, the steering angle sensor 14 is not essential, it does not need to be provided, and it is possible to obtain the steering angle from a rotation sensor such as a resolver connected to the motor 20.
A controller area network (CAN) 100 exchanging various information of a vehicle is connected to the control unit 30, and it is possible to receive the vehicle speed Vs from the CAN 100. Further, it is also possible to connect a non-CAN 101 exchanging a communication, analog/digital signals, a radio wave or the like except with the CAN 100 to the control unit 30.
The control unit 30 mainly comprises an MCU (including a CPU, an MPU and so on), and general functions performed by programs within the MCU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Ts detected by the torque sensor 10 and the vehicle speed Vs detected by the vehicle speed sensor 12 (or from the CAN 100) are inputted into a current command value calculating section 31 that calculates a current command value ‘ref’. The current command value calculating section 31 calculates the current command value Iref1 that is a control target value of a motor current supplied to the motor 20 on the basis of the inputted steering torque Ts and the inputted vehicle speed Vs and by using an assist map or the like. The current command value ‘ref’ is inputted into a current limiting section 33 through an adding section 32A. A current command value Irefm the maximum current of which is limited is inputted into a subtracting section 32B, and a deviation I (=Irefm−Im) between the current command value Irefm and a motor current value Im being fed back is calculated. The deviation I is inputted into a proportional integral (PI) control section 35 for improving a characteristic of the steering operation. The voltage control command value Vref whose characteristic is improved by the PI-control section 35 is inputted into a PWM-control section 36. Furthermore, the motor 20 is PWM-driven through an inverter 37 serving as a driving section. The motor current value Im of the motor 20 is detected by a motor current detector 38 and is fed back to the subtracting section 32B. The inverter 37 uses field effect transistors (FETs) as driving elements and is comprised of a bridge circuit of FETs.
A compensation signal CM from a compensation signal generating section 34 is added to the adding section 32A, and a characteristic compensation of the steering system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensation signal generating section 34 adds a self-aligning torque (SAT) 34-3 and an inertia 34-2 at an adding section 34-4, further adds the result of addition performed at the adding section 34-4 with a convergence 34-1 at an adding section 34-5, and then outputs the result of addition performed at the adding section 34-5 as the compensation signal CM.
In such an electric power steering apparatus, on the one hand it is desired to set responsibility of a current control system high in order to improve sense of unity of a vehicle and driver's steering, but on the other hand it is desired to enhance noise immunity in order to reduce a sound and a vibration that are caused by a detection noise included in a steering torque obtained from a sensor and a detected motor current value. However, it is generally difficult to make these two compatible because the noise immunity decreases when the responsibility of the current control system is made high.
As a method to solve this, for example, two-degree-of-freedom control has been utilized. The two-degree-of-freedom control is a control system capable of independently setting two control characteristics that one is a feedback characteristic such as robust stability and a disturbance removal characteristic and the other is an output response characteristic to a target value (a target value response characteristic). The two-degree-of-freedom control is constituted of two elements, a feedback control element and a feedforward control element. The feedback characteristic is set by the element of the former, and the target value response characteristic is set by the element of the latter. In applying the two-degree-of-freedom control to an electric power steering apparatus, it is possible to individually set a target value response characteristic from a current command value to a motor current value and a feedback characteristic of a feedback mechanism. Both this target value response characteristic and this feedback characteristic affect the responsibility and the noise immunity, in particular, the target value response characteristic greatly contributes the responsibility, and the feedback characteristic greatly contributes the noise immunity, so that setting these characteristics individually enables compatibility of mutually exclusive performances.
The control method utilizing the two-degree-of-freedom control has been proposed, for example, in the publication of Japanese Patent No. 5034633 B2 (Patent Document 1). The method in Patent Document 1 makes the responsibility and the noise immunity compatible to a high degree by considering a delay of operation time in a coefficient of a controller used as a feedforward control element and a feedback control element, and configuring a controller (a feedback control element) in a closed loop with two degrees or more.
However, a demand for performance of the controller has been upgraded year after year, and even if such a method as in Patent Document 1 makes the responsibility and the noise immunity compatible to a high degree, there is the case where a response is insufficient since a demanded performance is changed in accordance with a state of an electric power steering apparatus (EPS). For example, in the majority range of vehicle speed, it is desired that a vehicle follows even minute steering rather than performance for a noise. Accordingly, in order to enhance the responsibility, it is desirable to set both a response frequency in the target value response characteristic (a command value response frequency) and a response frequency in the feedback characteristic (a closed loop response frequency) high. On the other hand, at a very low vehicle speed such as during stop and during creep travelling, sensitivity to the above sound and the vibration that are caused by the detection noise increases, in particular, they are notably felt in steering holding, so that it is necessary to enhance the noise immunity, and it is desirable to set the closed loop response frequency low. Thus, the performance required in states of the vehicle speed is in the relation of trade-off. Though the control method to which a function of adjusting a gain of the feedback characteristic on the basis of a motor angular velocity being one of EPS states is added, is proposed in Patent Document 1, configuring a controller that is capable of changing a characteristic in accordance with the EPS state with a higher degree of freedom, is needed.
There is, for example, a method proposed in the publication of Japanese Patent No. 5548645 B2 (Patent Document 2) as a method of changing the characteristic of the controller according to the EPS state. The method in Patent Document 2 changes the characteristic of the controller according to the EPS state by determining a correction gain depending on a vehicle state or a steering state, and correcting a proportional (P) gain and an integral (I) gain of a d-axis current controller.